The present invention relates in general to resonant cavity filter structures and more particularly to such cavities that collectively form a duplexer useful in radio communication apparatus and wherein such cavity structures minimize the generation of wideband noise, eliminates coarse adjustments and reduces the sensitivity of the resonant frequency of the resultant cavity structure to vibration.
Resonant cavity structures are of course known in the art. So too are duplexer devices which incorporate one or more of such resonant cavities. Such duplexers are primarily intended for use in radio communication apparatus to selectively pass signal energy between the transmitter and an antenna and between the receiver and the same antenna. For operating frequencies in the UHF frequency band range, for example, these cavities are on the order of 20 cubic inches or so and are capable of passing substantial power or signal energy.
Conventionally, these cavity structures include a foreshortened resonator element in the interior thereof to determine the final resonant frequency the cavity will operate at. Typically the resonator is at or about one-eighth of a wave length of the involved frequency or frequency range. A tuning element typically extends through the cavity wall into close proximity of the resonator and by adjusting the position thereof, the end loading of the resonator is altered and with it the resonant frequency of the cavity. In this way, the cavity becomes tunable over a certain predictable frequency range. This tuning element must be accessible from the exterior of the cavity and therefore is made threadable to mate with corresponding machine screw threads in the wall of the cavity housing, usually in the top thereof.
Therein lies the difficulty for those cavity devices known in the art. This thread interface gives rise to a number of deleterious effects on desired performance. RF currents must passs through these threads and because of the inevitable minute imperfections therein, the metal-to-metal contact is only at a few point contacts within the thread with oxide "insulators" therebetween. At relatively high power levels, transmission line effects that occur within these threads give rise to a potential difference across the oxides. The magnitudes thereof are entirely sufficient to break down the oxides with the result that electric arcs occur which in turn gives rise to the generation of wide band noise. In the receiver portion of the associated transceiver, "desensing" occurs, and in the transmitter section power is lost. This phenomenon is known in the art as "fritting", and because the arcs tend to be mechanically unstable, the generation of noise is sustained for long periods of time, all to the detriment of the performance of the associated transceiver. Further, the lack of consistent low impedance connection in the interface threads themselves result in reduced unloaded Q for the cavity and resonant frequency sensitivity with vibration causing intermittent power out.
These problems have been addressed in the past, but those heretofore attempted solutions have proved unsatisfactory in one aspect or another. One prior approach involved a lock or jam nut on the threadable tuning element intended to rigidly hold such element in its final adjusted position. However, such does not prevent oxides from forming and the attendant problems therein. Also such jam nuts often cause distortion in the threads themselves.
Accordingly, what is needed is a substantial improvement or enhancement in the thread contacts where ground return is effected so as to eliminate or substantially reduce the generation of wideband noise due to fritting and to accommodate any minute thread imperfections that may otherwise occur. This, in turn, would optimize unloaded Q and make the resonant frequency of the cavity stable with vibration.